One of the advantages of fuel cells is that they can, in principle, convert the chemical energy of fuels directly into electrical energy at high efficiencies. In practice, however, some of the energy is always “lost” irreversibly as heat and unused fuel. Since the electric power is the most valuable output, one of the most important characteristics of a fuel cell design is the percentage of the available energy of the fuel that is converted into electricity. High electrical conversion efficiency requires that both the thermodynamic efficiency and fuel utilization be high. Thermodynamic efficiency is an intrinsic property of the energy conversion device, and depends on the reaction steps by which fuels undergo oxidation. In general, fuel cells have the advantage of higher thermodynamic efficiency than conventional heat engines. Fuel utilization determines how much of the fuel entering the device is actually converted into carbon dioxide and water vapor. Most fuel cells designed to operate directly on hydrocarbon fuels suffer from poor fuel utilization. Currently, diesel-electric generators are available that convert about 40% of the heat content of diesel fuel into electricity. Gas turbine/electric generators have a practical upper limit of about 50%. Thus, fuel cells designed to operate directly on hydrocarbon fuels must exceed 50% net electrical efficiency in order to be commercially viable for distributed electric power generation in direct competition with centralized utilities.
Fuel cells have been proposed for many applications including stationary electric power generation and electrical vehicular power plants to replace internal combustion engines. Hydrogen is often used as the fuel and is supplied to the fuel cell's anode. Oxygen (typically as air) is the cell's oxidant and is supplied to the cell's cathode. Hydrogen used in the fuel cell can be derived from the reformation of natural gas (methane), propane, methanol, ethanol or other hydrocarbon fuels. Complete conversion of hydrocarbon fuels to carbon dioxide and hydrogen requires a 2-step process. First, the fuel is steam reformed by reaction with water vapor (steam) to produce carbon monoxide and hydrogen. This reaction is given below for methane, but may be generalized for any hydrocarbon.CH4(g)+H2O(g)→CO(g)+3H2(g)   (1.1)Next, the carbon monoxide is reacted with additional steam by the “water gas shift” reaction,CO(g)+H2O(g)→CO2(g)+H2(g)   (1.2)These reactions are accomplished heterogeneously at catalytically active surfaces within a chemical reactor. The chemical reactor provides the necessary thermal energy throughout the catalyst to yield a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide, and water vapor, depending on the chemical equilibrium. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh.
In certain fuel cells operating directly on hydrocarbon fuels at elevated temperatures, it is not necessary to convert the fuel into hydrogen beforehand. The reforming and shift reactions are carried out in the cell, at or near the anode. Steam is injected into the gaseous fuel stream entering the cell. Only two water molecules are actually required for each carbon in the fuel in order to complete the reactions, but typically a higher steam to carbon ratio is used to enhance the production of the desired reaction product, hydrogen. High temperature fuel cells operating directly on hydrocarbon fuels are plagued by the propensity of hydrocarbons to pyrolize spontaneously on the catalyst into atomic carbon and hydrogen. This process, called “coking”, occurs when there is insufficient steam in the immediate vicinity of any hydrocarbons adsorbed on the surface of the catalyst to complete the hydrocarbon reforming reaction before pyrolysis occurs. These carbon deposits foul, and eventually destroy, the cell.
Recently, certain fuel cells based on oxide ion conducting electrolyte ceramic membranes have been developed that oxidize hydrocarbon fuels directly at the anode, without the need to supply externally injected steam. That is, dry hydrocarbon fuels at the anode react with oxygen ions passing through the electrolyte to produce carbon dioxide and water vapor directly. These fuel cells have the advantage of higher thermodynamic efficiency and greater simplicity than other intermediate and high temperature fuel cells, but still suffer from the problems of coking and poor fuel utilization.